Neodymium glass laser having an output at 904 nm.

ABSTRACT

A LASERABLE GLASS MATERIAL DOPED WITH A QUANTITY OF NEODYMIUM IONS IN A LOW CONCENTRATION RESULTS IN THE GLASS EXHIBITING A RATIO OF FLUORESCENT INTENSITY PEAKED AT 904 NANOMERS OVER THE FLUORESCENT INTENSITY PEAKED AT APPROXIMATELY 1060 NANOMETERS OF GREATER THAN 0.4 AS MEASURED BY A CARY MODEL 14 SPECTROPHOTOMETER. THE GLASS ENABLES THE GENERATION OF LASER LIGHT IN A WAVEBAND WITH AN OPTICAL CENTER AT ABOUT 904 NANOMETERS WHEN POSITIONED IN A LASER CAVITY WHICH IS RESONANT AT 904 NANOMETERS.

United States Patent 3,808,146 NEODYMIUM GLASS LASER HAVING AN OUTPUT AT904 NM.

Charles C. Robinson and Robert R. Shaw, Sturbridge, Mass, assignors toAmerican Optical Corporation, Southbridge, Mass.

Continuation-impart of application Ser. No. 122,723, Mar. 10, 1971. Thisapplication Jan. 22, 1973, Ser. No.

Int. Cl. C03c 3/28; C09k 1/66 U.S. Cl. 252301.4 R 1 Claim ABSTRACT OFTHE DISCLOSURE CROSSPREFERENCES TO RELATED APPLICATIONS This applicationis a continuation-in-part of our copending application Ser. No. 122,723filed Mar. 10, 1971, now Pat. No. 3,714,059, entitled Neodymium GlassLaser Having Room Temperature Output at Wavelengths Shorter Than 1060nm.

BACKGROUND OF THE INVENTION For many applications it is considereddesirable to have a laser device capable of producing an output of laserlight energy at wavelengths of approximately 900 nonometers. Thedesirability of generating light at this wavelength is notable withsystems utilizing detectors since there are detectors available whichare extremely sensitive at this Wavelength. Crystals exhibiting suchemission are known. For example, a YAG crystal laser is described in anarticle entitled, Oscillation and Doubling of the 0.946 4 Line in Nd+:YAG which appeared in Applied Physics Letter, vol. 15, No. 4, Aug. 15,1969, p. 111. A problem, however, with the YAG laser is that it is acrystal and thus does not possess the numerous advantages that are knownto be attendant with glass lasers.

Glass has various characteristics which can make it an ideal laser hostmaterial. It can be made in large pieces of diffraction-limited opticalquality, e.g., with an index refraction variation of less than one partper million across a 2.5-cm. diameter. In addition, glass lasers havebeen made in a variety of shapes and sizes from fibers a few micronswide supporting only a single dielectric waveguide mode, to rods 2meters long or 7.5 cm. in diameter. Furthermore, pieces of glass withquite different optical properties can be fused to solve certain systemdesign problems.

Glass composition can be tailored to give an index of refraction in therange of 1.5 to 2.0. Also, thermally stable laser cavities can beachieved by adjusting glass constituents to create an athermal laserglass.

There are two important diiferences between glass and crystal lasers.First, the thermal conductivity of glass is considerably lower than thatof most crystal hosts. The second important diflerence between glass andcrystal lasers is the inherently broader absorption and emission linesof ions in glass. These broader lines imply greater pump-lightabsorption, greater energy storage and much reduced spontaneousself-depletion for a given energy storage.

3,808,146 Patented Apr. 30, 1974 SUMMARY OF THE INVENTION In accordancewith the present invention a neodymium doped laser glass is providedwhich enables generation of laser radiation at about 904 nanometers.

Accordingly, it is an object of the present invention to provide newneodymium doped laser glass devices which generate laser light energy inwaveband with an optical center at about 904 nanometers.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is an emission curve showing thefluorescent emission properties of the glass utilized in laser devicesof the present invention;

FIG. 2 is a schematic representation of the various energy levels in aNd ion in the low soda borate glass of the present invention;

FIG. 3 is a diagrammatic illustration of a laser device of the presentinvention; and

FIG. 4 is a transmittance and reflectance curve of a reflector useful inthe laser device of the present invention.

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DESCRIPTION OF THE PREFERRED EMBODIMENTS In accordance with the presentinvention, a, laser device is provided which is comprised of a neodymiumdoped glass laser host positioned within an optically regenerative lasercavity. It has been found that trivalent neodymium ions in glass hoststypically have emission curves of the general shape shown in FIG. 1.This curve is provided at the outset to illustrate properties which areuseful in carrying out the object of the present invention.

The fluorescence cur-ves shown were measured in a Cary 14spectrophotometer by placing the glass sample in a copper fixture whichin turn was placed in the sample compartment of the Cary. The glass wasirradiated at right angles with a xenon arc lamp through a filter whichblocked the transmission of wavelengths longer than approximately 800nm. The fluorescent spectrum was recorded using the automatic slitcontrol which adjusted the slit width so that the output of a coiledtungsten filament lamp with a filament temperature of approximately2800" K. produced a constant deflection on the recording chart for allwavelengths. Thus the recording chart must be corrected to obtain thetrue relative intensities by dividing the chart deflection by a factorproportional to the energy radiated by the tungsten lamp at thewavelengths of interest. We have estimated the correction factor forobtaining the ratio of the 940 nm. fluorescent intensity to the 1060 nm.intensity to be approximately unity. This estimate was made by using thetungsten emissivities measured by I. C. DeVos (I. C. DeVos, Physics20,690 (1954)) for a ribbon filament tungsten lamp operating at 2800" K.in a calculation of the energy radiated by the coiled filament lamp atthe two wavelengths of interest. The intensity ratios reported here weremeasured directly from the Cary charts using no correction factor.

In FIG. 1 a curve 10 is shown with peaks 12 and 14 at 904 nanometers and1060 nanometers respectively. Curve 10 was measured at room temperature,300 K. A second curve 10 was measured 1.3 K. and has similar peaks 12'and 14. In connection with these and similar peaks, it is to beunderstood that the actual range of useful fluorescent emission issomewhat broad. In fact, in accordance with the invention, peaks 12 and12' can have a band width of 20 nanometers located between 890-910nanometers, while peak 14 can have a width of 20 nanometers locatedbetween 1050-1070 nanometers. Although curve 10 shows other peaks, forpurposes of the present invention the entire peaks represented by peaks3 12/12 and 14/14 of curve are the most critical. Tests have indicatedthat when a neodymium doped glass host is positioned in a cavity withreflectors that suppress emission at 1060 nanometers, peaks 12/12 and14/14 are the only peaks that need be considered in evaluating whetherthe laser will emit at 1060 nm. or 904 nm. It has been determined that aratio of peak intensities of at least .4 produces operative results atroom temperature, that is, laser action at 904 nanometers at roomtemperature (20 -C.), it is to be understood that in accordance with theinvention the greater the magnitude of the foregoing ratio the moreeffective will be the host for producing the desired laser emission whenpositioned in the cavity of the present invention whether at roomtemperature or at a lower temperature.

As indicated above, in addition to considering the emission spectra ofthe host glass, consideration must also be given to the opticallyregenerative laser cavity into which the host glass is positioned. Inaccordance with the invention the reflectors forming the laser cavitymust suppress laser emission at 1060 nanometers. It is to be understoodthat such reflectors are available and that the re flectors per se formno part of the present invention. For example, dichroic reflectors areavailable which transmit approximately 85% of the light at 1060nanometers while reflecting approximately 99.7% of the light between therange of 800-1000 nanometers.

Although not intended to be restricted to a particular theory, anunderstanding of the energy level scheme including the I manifold of theNd ion is useful in explaining the present invention. In this regard,FIG. 2 is' provided as a schematic representation of the various energylevels in a Nd ion.

A condition necessary for laser action according to this invention isthat the population of the initial state be at least as large as theterminal state which requires, therefore, that the initial statepopulation be at least 0.033 of the total population in the groundmanifold I The cavity losses for the 1060 nanometers emission must behigher than those for the 904 nanometers emission including the effectof the population in level 2.

In accordance with the invention, a laser glass has the composition inpercent by weight as set forth in the following table.

A rod 30 of the laser glass is positioned in an optically regenerativelaser cavity formed by reflectors 32 and 34, as is shown in FIG. 3. Thereflector 32 is 98% reflective for light at 904 nanometers and 98.4%reflective at 1060 nanometers. The second reflector 34 is 99.5%reflective at 904 nanometers and reflective at 1060 nanometers. A pumplight source is not shown in FIG. 3, it being understood that many pumpsources are available which will produce the required populationinversion in the neodymium ions. One such pump source commonly employedis a xenon flash tube. In this regard, the hardware for producing energyinversions are conventional and form no part of the present invention.

It can be seen by examining the curves of FIG. 1, that the ratio ofintensity peaks 12/14, i.e. at room temperature, is approximately 0.5;while at 1.3K, the ratio 12714 is improved to 0.6.

The transmittance and reflectance curve of the type reflector used forreflector 34 is shown in FIG. 4 of the drawing. Such a reflector isavailable from- Spectra- Physics, 1250 West Middlefield Road, MountainView, Calif. 94040.

The laser glass, as set forth in the foregoing table, is preferablyformed in the following manner. The components are added to the batch asH BO Na CO and Nd O The constituents are added in the known stoichiometric amounts to yield a glass having a final composition as set forthin the foregoing table. The glass making raw materials must be of highpurity and, in particular, must be free of contamination from iron orother elements which would cause light absorption at the desired laseremission wavelength if they were present in the finished glass. Thefinished glass, for example, should not contain more than 5 parts permillion of iron as Fe O The glass may be prepared by fusing the rawmaterials in a platinum crucible heated in a Globar electric furnace oran RF induction coil. No special atmosphere is necessary in the furnace.The raw materials are mixed intimately and as completely as possible ina mixing device that does not introduce any contamination. The mixedbatch is located into a platinum crucible which will not contaminate themelt with undesirable impurities. The crucible should be raised to atemperature of approximate- 1y 1000 C. The batch is held at thistemperature for approximately 15 minutes for a 50 gram sample. Duringthis time, the melt is stirred with a platinum rod. The glass is thencast in a cast iron mold at room temperature. It is to be understoodthat larger batches require a different procedure.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription, and all changes which come within the meaning and range ofequivalency of the claims are therefore intended to be embraced therein.

What we claim is:

1. A fluorescent glass material which fluoresces at wavelengths with anoptical center at approximately 904 nanometers and wavelengths with anoptical center at approximately 1060 nanometers where the ratio of therelative fluorescent intensity peaked at approximately 904 nanometersover the relative fluorescent intensity peaked at approximately 1060nanometers is at least .4 at room temperature, and where the materialconsists essentially of the following composition as given in weightpercent OTHER REFERENCES 'Morey: The Properties of Glass, ReinholdPublishng Co. (1957), p. 236.

EDWARD J. MEROS, Primary Examiner I. COOPER, Assistant Examiner US. Cl.X.R. 106--47 R UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTIONPatent No. 3 ,808, 146 Dated April 30, 1.974

Inventor(s) Charles C. Robinson and Robert R. Shaw It is certified thaterror appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

I1 column 1, at line 8, delete the numbers "321,997" and insert therefor--32l,907--.

Signed and sealed this 10th day of September 197s.

(SEAL) Attest:

MCCOY M. GIBSON, JR. C. MARSHALL DANN Attesting Officer Commissioner ofPatents USCOMM-DC 60375-P69 U. Sv GOVERNMENT PRINTING OFFICE: 19690-366-334.

FORM PO-IOSO (10-69)

